This application Claims benefit of U.S. Provisional Patent Application No. 60/285,221, filed Apr. 20, 2001
1. Field of the Invention
The present invention relates to polarization insensitive tunable optical filters having retained complementary outputs.
2. Description of the Prior Art
Optical fiber wavelength division multiplexed (WDM) communications systems are theoretically capable of extremely high data rates (terabits per second), meaning that many channels of gigabit rate data can theoretically be carried on a fiber, via wavelengt h division multiplexing.
Currently the two methods of constructing nodes on a fiber network are Optical to Electronic to Optical (OEO) conversion and fixed optical add/drop filters. OEO is the most common method, but is very expensive. The signals which are not being dropped are used to modulate lasers and the resulting wavelengths are multiplexed back in. Much of the hardware is data rate dependant.
Fixed optical add/drop nodes are simpler and less expensive, but must be replaced when any change is made. In addition, certain paths through the network are blocked from use, as no single wavelength can connect them. If the network is manually configured to remove a given block, another blocked path is inevitably created. This problem grows rapidly with increasing network complexity.
The utility of fiber optic systems has been limited because a truly useful optical tunable add/drop filter requires five characteristics:
1) Flat-topped pass bands, so that the modulation sidebands of the signal (where all of the information resides) are not attenuated;
2) Accurate tunability;
3) Retaining of the complementary output so the filter can add and/or drop, and those signals which do not emerge from one of the outputs will emerge from the other;
4) Hitless tuning, so that the filter is able to switch from dropping or adding any channel to any other channel without interference to other channels; and
5) Polarization insensitivity, so that the filter does not have differing bandpasses or optical path lengths for inputs of differing polarizations.
Currently, optical add/drop filters lacking at least one of the above characteristics are used to extract desired frequencies. Most tunable filter technologies, such as acousto-optic filters and Fabre-Perot filters cannot be constructed with flat-topped pass bands.
Interference filters are a relatively inexpensive, mature technology, and produce a flat bandpass. It is common to get flat-topped pass-bands and channel spacings down to 100 GHz (0.8 nm, in the 1550 nm communications band). Interestingly, interference filters can theoretically be tuned across a significant bandwidth by changing the angle of incidence of the light striking the filter. The limitation formerly preventing interference filter systems from being both tuned (rotated) and used as add/drop filter was the great difficulty of tracking the reflected output) as the filter was rotated. U.S. Pat. No. 6,362,904 by the present inventor (incorporated herein by reference) illustrates configurations which overcome this limitation. However, these configurations generally require a polarized signal or external circuitry to render the filter embodiments polarization insensitive.
A need remains in the art for tunable optical filters which retain the complementary output, have flat topped pass bands, and are polarization insensitive.
An object of the invention is to provide tunable optical filters which retain the complementary output, have flat topped pass bands, and are polarization insensitive.
A polarization independent tunable optical filter system according to the present invention includes a filter of the type which reflects a pass signal and transmits a drop signal according to frequency, tunable by rotation with respect to the input beam, and a retro reflector assembly situated and aligned to intercept light reflected from the filter. The retro reflector assembly includes a quarter wave plate, and a retro reflector element. The pass signal reflected off the filter a first time passes through the quarter wave plate a first time, reflects off of the retro reflector element, passes through the quarter wave plate a second time, and reflects off of the filter a second time.
In general, the filter is an interference filter. Preferably, the system includes a null state/all-pass element. In this case, the filter has a mirror portion adjacent to a filtering portion. The lens is a cylindrical lens whose axis is parallel to the the direction from the mirror portion to the filtering portion. A beam displacer selectively redirects the input beam toward the mirror portion or the filtering portion.
The retro reflector element might be a lens located one focal length from the filter and a mirror located one focal length from the lens. Or, the retro-reflector element could be an array of retro-reflector devices (such as corner cubes or cat""s eye lenses). Another retro-reflector element configuration is a first mirror affixed adjacent to the filter, wherein the fixed angle formed by the plane of the filter and the plane of the first mirror is under 180xc2x0, so that the the mirror and the filter forming a reflector assembly. The input beam is directed such that the portion of the input beam that reflects off of the interference filter also reflects off of the mirror. The reflector assembly is rotated about an axis at the vertex of the the plane of the filter and the plane of the mirror to tune the filter. A second mirror is aligned to intercept the reflected light from the first mirror, and the quarter wave plate is located between the first mirror and the second mirror. In the reflector assembly configuration, the angle formed by the plane of the filter and the plane of the first mirror might be approximately 45xc2x0.
A circulator between the input beam and the filter can provide the input beam to the filter, and also provide light reflected from the filter the second time as a pass signal. Another circulator between the filter and the drop signal can provide the input light transmitted through the filter as a drop signal and can provide an add signal to the filter for combination with the pass signal.
Alternatively, a two fiber connector between the input beam and the filter may provide the input beam along a first path to the filter, and collect light reflected from the filter the second time as a pass signal along a second path. In this case, a second connector between the filter and the drop signal collects light passed through the filter along a first path and for provides an add signal to the filter along a second path.